Phenolic Compounds and Their Fates In Tropical Lepidopteran Larvae: Modifications In Alkaline Conditions
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Lepidopteran larvae encounter a variety of phenolic compounds while consuming their host plants. Some phenolics may oxidize under alkaline conditions prevailing in the larval guts, and the oxidation products may cause oxidative stress to the larvae. In this study, we aimed to find new ways to predict how phenolic compounds may be modified in the guts of herbivorous larvae. To do so, we studied the ease of oxidation of phenolic compounds from 12 tropical tree species. The leaf extracts were incubated in vitro in alkaline conditions, and the loss of total phenolics during incubation was used to estimate the oxidizability of extracts. The phenolic profiles of the leaf extracts before and after incubation were compared, revealing that some phenolic compounds were depleted during incubation. The leaves of the 12 tree species were each fed to 12 species of lepidopteran larvae that naturally feed on these trees. The phenolic profiles of larval frass were compared to those of in vitro incubated leaf extracts. These comparisons showed that the phenolic profiles of alkali-treated samples and frass samples were similar in many cases. This suggested that certain phenolics, such as ellagitannins, proanthocyanidins, and galloylquinic acid derivatives were modified by the alkaline pH of the larval gut. In other cases, the chromatographic profiles of frass and in vitro incubated leaf extracts were not similar, and new modifications of phenolics were detected in the frass. We conclude that the actual fates of phenolics in vivo are often more complicated than can be predicted by a simple in vitro method.
KeywordsAmazonia Faeces MRM Neotropical Oxidation Rain forest SRM UHPLC–DAD–MS
The authors thank field workers in Peru, especially Lidia Sulca and Jair Reategui. Gerardo Lamas helped to identify the Lepidopteran species. Marica Engström and Maija Pälijärvi created the MS/MS quantitation method, and Paul Ek ran the ICP–MS analysis. Funding from the Kone Foundation (Biodiversity and multitrophic interactions project), University of Turku Graduate School to MV and IG, and Academy of Finland grant 258992 to JPS are acknowledged. The following Peruvian institutions provided study permits and help during the field studies: Universidad Nacional de la Amazonía Peruana (UNAP), Instituto de Investigaciones de la Amazonía Peruana (IIAP), Universidad Nacional Mayor de San Marcos and Ministerio de Agricultura.
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